Method and apparatus for design of a metrology target

ABSTRACT

A method and apparatus are described for providing an accurate and robust measurement of a lithographic characteristic or metrology parameter. The method includes providing a range or a plurality of values for each of a plurality of metrology parameters of a metrology target, providing a constraint for each of the plurality of metrology parameters, and calculating, by a processor to optimize/modify these parameters within the range of the plurality of values, resulting in a plurality of metrology target designs having metrology parameters meeting the constraints.

This application is a continuation of U.S. patent application Ser. No.15/650,401, filed Jul. 14, 2017, which claims benefit of U.S.Provisional Patent Application No. 62/362,812, filed Jul. 15, 2016 andU.S. Provisional Application 62/385,615, filed Sep. 9, 2016, and areincorporated herein in their entirety by reference.

FIELD

The present description relates to methods and apparatus to determineone or more structural parameters of a metrology target usable, forexample, in the manufacture of devices by a lithographic technique andto methods of manufacturing using a lithographic technique.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In lithographic processes, it is desirable to frequently makemeasurements of the structures created, e.g., for process control andverification. One or more parameters of the structures are typicallymeasured or determined, for example the overlay error between successivelayers formed in or on the substrate. There are various techniques formaking measurements of the microscopic structures formed in alithographic process. Various tools for making such measurements areknown, including scanning electron microscopes, which are often used tomeasure critical dimension (CD), and specialized tools to measureoverlay, the accuracy of alignment of two layers in a device. An exampleof such a tool is a scatterometer developed for use in the lithographicfield. This device directs a beam of radiation onto a target on thesurface of the substrate and measures one or more properties of theredirected radiation—e.g., intensity at a single angle of reflection asa function of wavelength; intensity at one or more wavelengths as afunction of reflected angle; or polarization as a function of reflectedangle—to obtain a “spectrum” from which a property of interest of thetarget can be determined. Determination of the property of interest maybe performed by various techniques: e.g., reconstruction of the targetstructure by iterative approaches such as rigorous coupled wave analysisor finite element methods, library searches, and principal componentanalysis.

SUMMARY

This invention relates to a method of designing a metrology target and amethod of measurement of a lithographic characteristic using a metrologytarget and a metrology apparatus.

Optical metrology uses light scattered from a target to provideinformation about a lithographic process. The measurements are performedin optical instruments such as scatterometers. The information thatscatterometers are suitable to measure is overlay, which is a relativedistance between two overlapping gratings, in a plane parallel with thetwo overlapping gratings.

In a diffraction based overlay measurement, the overlay is extractedfrom a difference in the light intensity for the first positive andnegative first diffracted order. The stack sensitivity is defined as aratio of overlay sensitivity K, which is a proportionality factorlinking measured light intensity and overlay OV, and averaged lightintensity Im, ratio multiplied by 20 nm.

Examples of known scatterometers include those scatterometers describedin US2006033921A1, US2010201963A1, US2006066855A1, US2014192338,US2011069292A1, US20110027704A, US20110043791A, US2011102753A1,US20120044470A, US20120123581A, US20130258310A, US20130271740A andWO2016083076A1. The contents of all these applications are specificallyand entirely incorporated herein by reference.

Further, it is desirable to be able to use a metrology target which hisselected such that, when used in a metrology measurement, it provides anoptimal and robust result, which in turn leads to accurate overlaymeasurement. More information about target design is in Appendix, whichis specifically and entirely incorporated herein by reference.

One of the problems faced by metrology applications of diffraction basedoverlay is that the stack sensitivity (one of the parameters of themetrology measurement process, i.e. a metrology parameter) isproportional to the wavelength of the light used when illuminating thetarget.

Said proportionality of the stack sensitivity with wavelength shows alsodecreased periodicity as a function of wavelength when the verticaldistance (thickness), between the gratings used to form the target,becomes larger.

Furthermore, the process of selecting and/or adjusting the light used inthe metrology process is difficult as it puts constraints on the type ofsource providing the light in illuminating the target and it puts alsoconstraints on the wavelength selecting means used in such metrologyapparatus.

It is desirable, for example, to provide methods and apparatus fordesign of a metrology target. Furthermore, although not limited to this,it would be of advantage if the methods and apparatus could be appliedto accurately measure and minimize overlay error in lithographicprocess.

An object of the current invention is to provide a method of accurateand robust measurement of a lithographic characteristic.

According to the present invention, there is provided a method ofmetrology target design, the method comprising: receiving anillumination parameter for measuring a metrology target and selectingand/or adjusting a metrology parameter associated with the metrologytarget design for enhancing an accuracy and/or a robustness of themeasurement of the metrology target design using the illuminationparameter.

Further, according to the present invention, there is provided a methodto determine a parameter of a litho process comprising: receiving thelight scattered from a region comprising at least two metrology targetsoptimized to provide a robust and optimal metrology measurement anddetermining the parameter of the litho process from a weightedcontribution of each individual metrology targets.

The illumination parameter is the wavelength or the polarization of theilluminating beam of the metrology apparatus, for example.

A metrology parameter is, for example, the pitch of the gratings used toform the metrology target. Another metrology parameter is the CD, theangle of the lines forming the gratings, the duty cycle of lines andspaces forming the grating.

In one embodiment of the invention, the pitch of the target is selectedand/or adjusted in metrology simulation package, such as Design forControl package, to have large K value of the overlay sensitivity whenthe target is illuminated with the radiation received from a user orselected in the metrology target design.

In another embodiment of the invention, a cluster of N targets isdesigned, by selecting and/or adjusting the pitch, CD, angle of thegrating forming lines, duty cycle of the line and spaces.

When illuminated with an illumination radiation, which has a wavelengthreceived and used in the design of the metrology target and/or is usedas a constraint in the design phase, the cluster of targets will provideat least N overlay sensitivity values, K_(i).

The simulation package selects and/or adjusts metrology parameters suchthat the weighted sum of K_(i) is maxim.

The weighs of each K_(i) are parameters α_(i). A further condition fordesign is that the sum of α_(i)=1 and −1<α_(i)<1.

A parameter of the lithographic process, such as overlay, is determined,for example, as a weighted sum of the overlay values measured from eachtarget wherein the weights are coefficients α_(i).

In another embodiment of the invention, a cluster of N targets isdesigned, by selecting and/or adjusting the pitch, CD, angle of thegrating forming lines, duty cycle of the line and spaces.

When illuminated with an illumination radiation, which has a wavelengthreceived and used in the design of the metrology target, the cluster oftargets will provide at least N overlay numbers, OV_(i).

The simulation package selects and/or adjusts metrology parameters suchthat the weighted sum of OV_(i) is maxim.

The weighs of each Ki are parameters α_(i) ^(OV).

The final overlay number is then a combination, for example linear, ofthe individual overlay numbers for the different targets.

The reference overlay number, which is the target to approach, isprovided by a self-referencing metrology method or it is provided from aCD-SEM measurement.

The weights α_(i) ^(OV) are not bound to [−1,1] interval of values.

The parameters α_(i) ^(OV) are determined, for example, from acorrelation analysis. An example of a correlation analysis is PCA(Principal Component Analysis).

A parameter of the lithographic process, such as overlay, is determined,for example, as a weighted sum of the overlay values measured from eachtarget wherein the weights are coefficients α_(i) ^(OV).

In an aspect of the present disclosure, there is provided a range or aplurality of values for each of a plurality of metrology parameters of ametrology target, providing a constraint for each of the plurality ofmetrology parameters, and calculating, by a processor to optimize theseparameters within the range of the plurality of values, resulting in aplurality of metrology target designs having metrology parametersmeeting the constraints.

In an aspect of the present disclosure, there is provided a methodcomprising measuring a modification value and a lithographic processparameter for each metrology target of a plurality of metrology targets,the plurality of metrology targets having been produced by metrologyparameters and a manufacturing process. The method further comprisesdetermining a multiplication factor for each metrology target based onits corresponding modification value, and determining an overalllithographic process parameter for the plurality of metrology targetsusing the modification value and the determined multiplication factors.

In an aspect of the present disclosure, there is provided a methodcomprising measuring a modification value and a lithographic processparameter for each metrology target of a plurality of metrology targets,the plurality of metrology targets having been produced by metrologyparameters and a manufacturing process. The method further comprisesdetermining a multiplication factor for each metrology target based onits corresponding modification value and a reference lithographicprocess parameter, and determining an overall lithographic processparameter for the plurality of metrology targets using the determinedmultiplication factors.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 schematically depicts an embodiment of a lithographic apparatus;

FIG. 2 schematically depicts an embodiment of a lithographic cell orcluster;

FIG. 3 schematically depicts an embodiment of a scatterometer;

FIG. 4 schematically depicts a further embodiment of a scatterometer;

FIG. 5 schematically depicts a form of multiple grating target and anoutline of a measurement spot on a substrate;

FIGS. 6A and 6B schematically depict a model structure of one period ofan overlay target showing an example of variation of the target fromideal, e.g., two types of process-induced asymmetry;

FIG. 7A, FIG. 7B and FIG. 7C respectively show schematic cross-sectionsof overlay periodic structures having different overlay values in theregion of zero;

FIG. 7D is a schematic cross-section of an overlay periodic structurehaving structural asymmetry in a bottom periodic structure due toprocessing effects;

FIG. 8 illustrates principles of overlay measurement in an ideal target,not subject to structural asymmetry;

FIG. 9 illustrates principles of overlay measurement in a non-idealtarget, with correction of structural asymmetry as disclosed inembodiments herein;

FIG. 10 illustrates an example graph of stack sensitivity as a functionof incident radiation wavelength for different incident illuminationpolarization;

FIG. 11 schematically illustrates a situation where no stack differenceexists between a first target periodic structure with a bias +d and asecond target periodic structure with a bias −d, and illustratesdiffraction signals following diffraction by the first and second targetperiodic structures;

FIG. 12 schematically illustrates the intensity variations of thecombined +1st diffraction order signal and the combined −1st diffractionorder signal diffracted by the first target periodic structure;

FIG. 13 schematically illustrates the intensity variations of thecombined +1st diffraction order signal and the combined −1st diffractionorder signal diffracted by the second target periodic structure;

FIG. 14 schematically illustrates a situation where a stack differenceexists between a first target periodic structure with a bias +d and asecond target periodic structure with a bias −d, and illustratesdiffraction signals following diffraction by the first and second targetperiodic structures;

FIG. 15 illustrates an example graph of stack sensitivity as a functionof incident radiation wavelength for different metrology target designs,according to an embodiment;

FIG. 16 is a flowchart of steps of a method for improving robustness andmeasurability of metrology target stacks, according to an embodiment;

FIG. 17A is a flowchart of steps of a method for improving robustnessand measurability of metrology target stacks, according to anotherembodiment;

FIG. 17B is an example graph of K value as a function of metrologytarget locations for different metrology target designs, according to anembodiment;

FIG. 18A is a flowchart of steps of a method for improving robustnessand measurability of metrology target stacks, according to a furtherembodiment;

FIG. 18B is an example graph of overlay value as a function of metrologytarget locations for different metrology target designs, according to anembodiment.

FIG. 19 is a flowchart of steps of a method for measuring a lithographyprocess parameter using metrology targets, according to an embodiment;

FIG. 20 is a flowchart of steps of a method for metrology systemcalibration using metrology targets, according to an embodiment;

FIG. 21 is a flowchart of steps of a method for designing metrologytargets, according to an embodiment.

FIG. 22 is a schematic illustration of a form of multi-grating metrologytargets, according to an embodiment.

DETAILED DESCRIPTION

Before describing embodiments in detail, it is instructive to present anexample environment in which embodiments may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatuscomprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. DUV radiation or EUV radiation);

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WTa constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support structure holds the patterning device in amanner that depends on the orientation of the patterning device, thedesign of the lithographic apparatus, and other conditions, such as forexample whether or not the patterning device is held in a vacuumenvironment. The patterning device support structure can use mechanical,vacuum, electrostatic or other clamping techniques to hold thepatterning device. The patterning device support structure may be aframe or a table, for example, which may be fixed or movable asrequired. The patterning device support structure may ensure that thepatterning device is at a desired position, for example with respect tothe projection system. Any use of the terms “reticle” or “mask” hereinmay be considered synonymous with the more general term “patterningdevice”.

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore tables (e.g., two or more substrate table, two or more patterningdevice support structures, or a substrate table and metrology table). Insuch “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask tableMT), and is patterned by the patterning device. Having traversed thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor IF (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WTa can be movedaccurately, e.g., so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g., mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe patterning device support (e.g., mask table) MT may be realized withthe aid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning), which form part of the first positioner PM.Similarly, movement of the substrate table WTa may be realized using along-stroke module and a short-stroke module, which form part of thesecond positioner PW. In the case of a stepper (as opposed to a scanner)the patterning device support (e.g., mask table) MT may be connected toa short-stroke actuator only, or may be fixed.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment markers may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. The alignment system, whichdetects the alignment markers is described further below.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the patterning device support (e.g., mask table) MT andthe substrate table WTa are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e., a single static exposure). The substratetable WTa is then shifted in the X and/or Y direction so that adifferent target portion C can be exposed. In step mode, the maximumsize of the exposure field limits the size of the target portion Cimaged in a single static exposure.

2. In scan mode, the patterning device support (e.g., mask table) MT andthe substrate table WTa are scanned synchronously while a patternimparted to the radiation beam is projected onto a target portion C(i.e., a single dynamic exposure). The velocity and direction of thesubstrate table WTa relative to the patterning device support (e.g.,mask table) MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the patterning device support (e.g., mask table) MTis kept essentially stationary holding a programmable patterning device,and the substrate table WTa is moved or scanned while a pattern impartedto the radiation beam is projected onto a target portion C. In thismode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after eachmovement of the substrate table WTa or in between successive radiationpulses during a scan. This mode of operation can be readily applied tomaskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Lithographic apparatus LA is of a so-called dual stage type which hastwo tables WTa, WTb (e.g., two substrate tables) and two stations—anexposure station and a measurement station—between which the tables canbe exchanged. For example, while a substrate on one table is beingexposed at the exposure station, another substrate can be loaded ontothe other substrate table at the measurement station and variouspreparatory steps carried out. The preparatory steps may include mappingthe surface control of the substrate using a level sensor LS andmeasuring the position of alignment markers on the substrate using analignment sensor AS, both sensors being supported by a reference frameRF. If the position sensor IF is not capable of measuring the positionof a table while it is at the measurement station as well as at theexposure station, a second position sensor may be provided to enable thepositions of the table to be tracked at both stations. As anotherexample, while a substrate on one table is being exposed at the exposurestation, another table without a substrate waits at the measurementstation (where optionally measurement activity may occur). This othertable has one or more measurement devices and may optionally have othertools (e.g., cleaning apparatus). When the substrate has completedexposure, the table without a substrate moves to the exposure station toperform, e.g., measurements and the table with the substrate moves to alocation (e.g., the measurement station) where the substrate is unloadedand another substrate is load. These multi-table arrangements enable asubstantial increase in the throughput of the apparatus.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to as a lithocell orlithocluster, which also includes apparatus to perform one or more pre-and post-exposure processes on a substrate. Conventionally these includeone or more spin coaters SC to deposit a resist layer, one or moredevelopers DE to develop exposed resist, one or more chill plates CH andone or more bake plates BK. A substrate handler, or robot, RO picks up asubstrate from input/output ports I/O1, I/O2, moves it between thedifferent process devices and delivers it to the loading bay LB of thelithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithographic controlunit LACU. Thus, the different apparatus may be operated to maximizethroughput and processing efficiency.

FIG. 3 depicts an embodiment of a scatterometer SM1. It comprises abroadband (white light) radiation projector 2 which projects radiationonto a substrate 6. The reflected radiation is passed to a spectrometerdetector 4, which measures a spectrum 10 (i.e. a measurement ofintensity as a function of wavelength) of the specular reflectedradiation. From this data, the structure or profile giving rise to thedetected spectrum may be reconstructed by processing unit PU, e.g. byRigorous Coupled Wave Analysis and non-linear regression or bycomparison with a library of simulated spectra as shown at the bottom ofFIG. 3. In general, for the reconstruction, the general form of thestructure is known and some parameters are assumed from knowledge of theprocess by which the structure was made, leaving only a few parametersof the structure to be determined from the scatterometry data. Such ascatterometer may be configured as a normal-incidence scatterometer oran oblique-incidence scatterometer.

Another embodiment of a scatterometer SM2 is shown in FIG. 4. In thisdevice, the radiation emitted by radiation source 2 is focused usinglens system 12 through interference filter 13 and polarizer 17,reflected by partially reflective surface 16 and is focused ontosubstrate W via a microscope objective lens 15, which has a highnumerical aperture (NA), desirably at least 0.9 or at least 0.95. Animmersion scatterometer may even have a lens with a numerical apertureover 1. The reflected radiation then transmits through partiallyreflective surface 16 into a detector 18 in order to have the scatterspectrum detected. The detector may be located in the back-projectedpupil plane 11, which is at the focal length of the lens 15, however thepupil plane may instead be re-imaged with auxiliary optics (not shown)onto the detector 18. The pupil plane is the plane in which the radialposition of radiation defines the angle of incidence and the angularposition defines the azimuth angle of the radiation. The detector isdesirably a two-dimensional detector so that a two-dimensional angularscatter spectrum (i.e. a measurement of intensity as a function of angleof scatter) of the substrate target can be measured. The detector 18 maybe, for example, an array of CCD or CMOS sensors, and may have anintegration time of, for example, 40 milliseconds per frame.

A reference beam is often used, for example, to measure the intensity ofthe incident radiation. To do this, when the radiation beam is incidenton the partially reflective surface 16 part of it is transmitted throughthe surface as a reference beam towards a reference mirror 14. Thereference beam is then projected onto a different part of the samedetector 18.

One or more interference filters 13 are available to select a wavelengthof interest in the range of, say, 405-790 nm or even lower, such as200-300 nm. The interference filter(s) may be tunable rather thancomprising a set of different filters. A grating could be used insteadof or in addition to one or more interference filters.

The detector 18 may measure the intensity of scattered radiation at asingle wavelength (or narrow wavelength range), the intensity separatelyat multiple wavelengths or the intensity integrated over a wavelengthrange. Further, the detector may separately measure the intensity oftransverse magnetic- (TM) and transverse electric- (TE) polarizedradiation and/or the phase difference between the transverse magnetic-and transverse electric-polarized radiation.

Using a broadband radiation source 2 (i.e. one with a wide range ofradiation frequencies or wavelengths—and therefore of colors) ispossible, which gives a large etendue, allowing the mixing of multiplewavelengths. The plurality of wavelengths in the broadband desirablyeach has a bandwidth of δλ and a spacing of at least 2δλ (i.e. twice thewavelength bandwidth). Several “sources” of radiation may be differentportions of an extended radiation source which have been split using,e.g., fiber bundles. In this way, angle resolved scatter spectra may bemeasured at multiple wavelengths in parallel. A 3-D spectrum (wavelengthand two different angles) may be measured, which contains moreinformation than a 2-D spectrum. This allows more information to bemeasured which increases metrology process robustness. This is describedin more detail in U.S. patent application publication no. US2006-0066855, which document is hereby incorporated in its entirety byreference.

By comparing one or more properties of the beam before and after it hasbeen redirected by the target, one or more properties of the substratemay be determined. This may be done, for example, by comparing theredirected beam with theoretical redirected beams calculated using amodel of the substrate and searching for the model that gives the bestfit between measured and calculated redirected beams. Typically aparameterized generic model is used and the parameters of the model, forexample width, height and sidewall angle of the pattern, are varieduntil the best match is obtained.

Two main types of scatterometers are used. A spectroscopic scatterometerdirects a broadband radiation beam onto the substrate and measures thespectrum (intensity as a function of wavelength) of the radiationscattered into a particular narrow angular range. An angularly resolvedscatterometer uses a monochromatic radiation beam and measures theintensity (or intensity ratio and phase difference in case of anellipsometric configuration) of the scattered radiation as a function ofangle. Alternatively, measurement signals of different wavelengths maybe measured separately and combined at an analysis stage. Polarizedradiation may be used to generate more than one spectrum from the samesubstrate.

In order to determine one or more parameters of the substrate, a bestmatch is typically found between the theoretical spectrum produced froma model of the substrate and the measured spectrum produced by theredirected beam as a function of either wavelength (spectroscopicscatterometer) or angle (angularly resolved scatterometer). To find thebest match there are various methods, which may be combined. Forexample, a first method is an iterative search method, where a first setof model parameters is used to calculate a first spectrum, a comparisonbeing made with the measured spectrum. Then a second set of modelparameters is selected, a second spectrum is calculated and a comparisonof the second spectrum is made with the measured spectrum. These stepsare repeated with the goal of finding the set of parameters that givesthe best matching spectrum. Typically, the information from thecomparison is used to steer the selection of the subsequent set ofparameters. This process is known as an iterative search technique. Themodel with the set of parameters that gives the best match is consideredto be the best description of the measured substrate.

A second method is to make a library of spectra, each spectrumcorresponding to a specific set of model parameters. Typically the setsof model parameters are chosen to cover all or almost all possiblevariations of substrate properties. The measured spectrum is compared tothe spectra in the library. Similarly to the iterative search method,the model with the set of parameters corresponding to the spectrum thatgives the best match is considered to be the best description of themeasured substrate. Interpolation techniques may be used to determinemore accurately the best set of parameters in this library searchtechnique.

In any method, sufficient data points (wavelengths and/or angles) in thecalculated spectrum should be used in order to enable an accurate match,typically between 80 up to 800 data points or more for each spectrum.Using an iterative method, each iteration for each parameter value wouldinvolve calculation at 80 or more data points. This is multiplied by thenumber of iterations needed to obtain the correct profile parameters.Thus many calculations may be required. In practice this leads to acompromise between accuracy and speed of processing. In the libraryapproach, there is a similar compromise between accuracy and the timerequired to set up the library.

In any of the scatterometers described above, the target on substrate Wmay be a grating which is printed such that after development, the barsare formed of solid resist lines. The bars may alternatively be etchedinto the substrate. The target pattern is chosen to be sensitive to aparameter of interest, such as focus, dose, overlay, chromaticaberration in the lithographic projection apparatus, etc., such thatvariation in the relevant parameter will manifest as variation in theprinted target. For example, the target pattern may be sensitive tochromatic aberration in the lithographic projection apparatus,particularly the projection system PL, and illumination symmetry and thepresence of such aberration will manifest itself in a variation in theprinted target pattern. Accordingly, the scatterometry data of theprinted target pattern is used to reconstruct the target pattern. Theparameters of the target pattern, such as line width and shape, may beinput to the reconstruction process, performed by a processing unit PU,from knowledge of the printing step and/or other scatterometryprocesses.

While embodiments of a scatterometer have been described herein, othertypes of metrology apparatus may be used in an embodiment. For example,a dark field metrology apparatus such as described in U.S. PatentApplication Publication No. 2013-0308142, which is incorporated hereinin its entirety by reference, may be used. Further, those other types ofmetrology apparatus may use a completely different technique thanscatterometry.

FIG. 5 depicts an example composite metrology target formed on asubstrate according to known practice. The composite target comprisesfour gratings 32, 33, 34, 35 positioned closely together so that theywill all be within a measurement spot 31 formed by the illumination beamof the metrology apparatus. The four targets thus are all simultaneouslyilluminated and simultaneously imaged on sensor 4, 18. In an examplededicated to overlay measurement, gratings 32, 33, 34, 35 are themselvescomposite gratings formed by overlying gratings that are patterned indifferent layers of the semiconductor device formed on substrate W.There may be a plurality of composite targets placed at differentlocations on substrate W such that measurements and information aboutthe entire substrate W can be obtained. Gratings 32, 33, 34, 35 may havedifferently biased overlay offsets in order to facilitate measurement ofoverlay between the layers in which the different parts of the compositegratings are formed. Gratings 32, 33, 34, 35 may also differ in theirorientation, as shown, so as to diffract incoming radiation in X and Ydirections. In one example, gratings 32 and 34 are X-direction gratingswith biases of +d, −d, respectively. This means that grating 32 has itsoverlying components arranged so that if they were both printed exactlyat their nominal locations, one of the components would be offsetrelative to the other by a distance d. Grating 34 has its componentsarranged so that if perfectly printed there would be an offset of d, butin the opposite direction to the first grating and so on. Gratings 33and 35 may be Y-direction gratings with offsets +d and −d respectively.While four gratings are illustrated, another embodiment may include alarger matrix to obtain desired accuracy. For example, a 3×3 array ofnine composite gratings may have biases −4d, −3d, −2d, −d, 0, +d, +2d,+3d, +4d. Separate images of these gratings can be identified in theimage captured by sensor 4, 18.

The metrology targets as described herein may be, for example, overlaytargets designed for use with a metrology tool such as Yieldstarstand-alone or integrated metrology tool, and/or alignment targets suchas those typically used with a TwinScan lithographic system, bothavailable from ASML.

In general, metrology targets for use with such systems should beprinted on the substrate with dimensions that meet the designspecification for the particular microelectronic device to be imaged onthat substrate. As processes continue to push against the limits oflithographic device imaging resolution in advanced process nodes, thedesign rule and process compatibility requirements place stress on theselection of appropriate targets. As the targets themselves become moreadvanced, often requiring the use of resolution enhancement technology,such as phase-shift patterning devices, and optical proximitycorrection, the printability of the target within the process designrules becomes less certain. As a result, proposed metrology targetdesign may be subject to testing and/or simulation in order to confirmtheir suitability and/or viability, both from a printability and adetectability standpoint. In a commercial environment, good overlay markdetectability may be considered to be a combination of low totalmeasurement uncertainty as well as a short move-acquire-move time, asslow acquisition is detrimental to total throughput for the productionline. Modern micro-diffraction-based-overlay targets (μDBO) may be onthe order of 10 μm on a side, which provides an inherently low detectionsignal compared to 40×160 μm² targets such as those used in the contextof monitor substrates.

Additionally, once metrology targets that meet the above criteria havebeen selected, there is a possibility that detectability will changewith respect to process variations, such as film thickness variation,various etch biases, and geometry asymmetries induced by the etch and/orpolish processes. Therefore, it may be useful to select a target thathas low detectability variation and low overlay/alignment variationagainst various process variations. Likewise, the fingerprint (printingcharacteristics, including, for example, lens aberration) of thespecific machine that is to be used to produce the microelectronicdevice to be imaged will, in general, affect the imaging and productionof the metrology targets. It may therefore be useful to ensure that themetrology targets are resistant to fingerprint effects, as some patternswill be more or less affected by a particular lithographic fingerprint.

FIGS. 6A and 6B schematically show a model structure of one period of anoverlay target showing an example of variation of the target from ideal,e.g., two types of process-induced asymmetry. With reference to FIG. 6A,the substrate W is patterned with a bottom grating 500, etched into asubstrate layer. The etch process used for the bottom grating results ina tilt of the floor 502 of the etched trench. This floor tilt, FT, canbe represented as a structural parameter, for example as a measure ofthe height drop across the floor 502, in nm. A BARC (bottomanti-reflective coating) layer 504 supports the patterned resist featureof the top grating 506. In this example, the alignment overlay errorbetween the top and bottom grating is zero, as the centers of the topand bottom grating features are at the same lateral position. However,the bottom-layer process-induced asymmetry, i.e. the floor tilt, leadsto an error in the measured overlay offset, in this case giving anon-zero overlay offset. FIG. 6B shows another type of bottom-layerprocess-induced asymmetry that can lead to an error in the measuredoverlay offset. This is side wall angle (SWA) unbalance, SWAun. Featuresin common with those of FIG. 6A are labeled the same. Here, one sidewall 508 of the bottom grating has a different slope to the other sidewall 510. This unbalance can be represented as a structural parameter,for example as a ratio of the two side wall angles relative to the planeof the substrate. Both asymmetry parameters floor tilt and SWA unbalancegive rise to an “apparent” overlay error between the top and bottomgratings. This apparent overlay error comes on top of the “real” overlayerror to be measured between the top and bottom gratings.

Accordingly, in an embodiment, it is desirable to simulate variousmetrology target designs in order to confirm the suitability and/orviability of one or more of the proposed target designs.

In the patent application publications mentioned above, varioustechniques are disclosed for improving the quality of overlaymeasurements using the basic method mentioned above. These techniqueswill not be explained here in further detail. They may be used incombination with the techniques newly disclosed in the presentapplication.

FIGS. 7A-7D show schematic cross-sections of target periodic structures(overlay periodic structures), with different bias offsets. These can beused as the target T on substrate W. Periodic structures withperiodicity in the X direction are shown for the sake of example only.Different combinations of these periodic structures with differentbiases and with different orientations can be provided separately or aspart of a target. Further details of the design of these periodic targetstructures are described in U.S. Patent Publication US 20150186582,which is hereby incorporated by reference in its entirety.

Starting with FIG. 7A, a target 600 formed in at least two layers,labeled L1 and L2, is shown. In the lower or bottom layer L1, a firstperiodic structure (the lower or bottom periodic structure), for examplea grating, is formed by features 602 and spaces 604 on a substrate 606.In layer L2, a second periodic structure, for example a grating, isformed by features 608 and spaces 610. (The cross-section is drawn suchthat the features 602, 608 (e.g., lines) extend into the page.) Theperiodic structure pattern repeats with a pitch P in both layers.Features 602 and 608 may take the form of lines, dots, blocks and viaholes. In the situation shown at FIG. 7A, there is no overlaycontribution due to misalignment, e.g., no overlay error and no imposedbias, so that each feature 608 of the second structure lies exactly overa feature 602 in the first structure.

At FIG. 7B, the same target with a first known imposed bias +d is shown,such that the features 608 of the first structure are shifted by adistance d to the right, relative to the features of the secondstructure. The bias distance d might be a few nanometers in practice,for example 10 nm-20 nm, while the pitch P is for example in the range300-1000 nm, for example 500 nm or 600 nm. At FIG. 7C, another featurewith a second known imposed bias −d, such that the features of 608 areshifted to the left, is depicted. The value of d need not be the samefor each structure. Biased periodic structures of this type shown atFIGS. 7A to 7C are described in the prior patent applicationpublications mentioned above.

FIG. 7D shows schematically a phenomenon of structural asymmetry, inthis case structural asymmetry in the first structure (bottom structureasymmetry). The features in the periodic structures at FIGS. 7A to 7C,are shown as perfectly square-sided, when a real feature would have someslope on the side, and a certain roughness. Nevertheless they areintended to be at least symmetrical in profile. The features 602 and/orspaces 604 at FIG. 7D in the first structure no longer have asymmetrical form at all, but rather have become distorted by one or moreprocessing steps. Thus, for example, a bottom surface of each space hasbecome tilted (bottom wall tilt). For example, side wall angles of thefeatures and spaces have become asymmetrical. As a result of this, theoverall target asymmetry of a target will comprise an overlaycontribution independent of structural asymmetry (i.e., an overlaycontribution due to misalignment of the first structure and secondstructure; itself comprised of overlay error and any known imposed bias)and a structural contribution due to this structural asymmetry in thetarget.

When overlay is measured by the method of FIG. 6 using only two biasedperiodic structures, the process-induced structural asymmetry cannot bedistinguished from the overlay contribution due to misalignment, andoverlay measurements (in particular to measure the undesired overlayerror) become unreliable as a result. Structural asymmetry in the firststructure (bottom periodic structure) of a target is a common form ofstructural asymmetry. It may originate, for example, in the substrateprocessing steps such as chemical-mechanical polishing (CMP), performedafter the first structure was originally formed.

In PCT patent application publication no. WO 2013-143814, it is proposedto use three or more component periodic structures to measure overlay bya modified version of the method of FIG. 6. Three or more periodicstructures of the type shown in FIGS. 7A to 7C are used to obtainoverlay measurements that are to some extent corrected for structuralasymmetry in the target periodic structures, such as is caused by bottomstructure asymmetry in a practical patterning process. However, thismethod requires a new target design (e.g. different to that illustratedin FIG. 4) and therefore a new patterning device or patterning devicepattern will be required. Furthermore, the target area is larger andtherefore consumes more substrate area. In addition, the phase elementof the overlay contribution resultant from structural asymmetry isignored in this and other prior methods, meaning that the correctionsare not as accurate as they could be if the phase element was alsocorrected for.

In FIG. 8 a curve 702 illustrates the relationship between overlay OVand intensity asymmetry A for an ‘ideal’ target having zero offset andno structural asymmetry within the individual periodic structuresforming the target, and in particular within the individual periodicstructure of the first structure. Consequently, the target asymmetry ofthis ideal target comprises only an overlay contribution due tomisalignment of the first structure and second structure resultant froma known imposed bias and overlay error OV. This graph, and the graph ofFIG. 9, illustrate the principles behind the disclosure only, and ineach graph, the units of intensity asymmetry A and overlay OV arearbitrary. Examples of actual dimensions will be given further below.

In the ‘ideal’ situation of FIG. 8, the curve 702 indicates that theintensity asymmetry A has a non-linear periodic relationship (e.g.,sinusoidal relationship) with the overlay. The period P of thesinusoidal variation corresponds to the period or pitch P of theperiodic structures, converted of course to an appropriate scale. Thesinusoidal form is pure in this example, but can include harmonics inreal circumstances.

As mentioned above, biased periodic structures (having a known imposedoverlay bias) can be used to measure overlay, rather than relying on asingle measurement. This bias has a known value defined in thepatterning device (e.g. a reticle) from which it was made, that servesas an on-substrate calibration of the overlay corresponding to themeasured intensity asymmetry. In the drawing, the calculation isillustrated graphically. In steps S1-S5, intensity asymmetrymeasurements A_(+d) and A_(−d) are obtained for periodic structureshaving imposed biases +d and −d respectively (as shown in FIG. 7B andFIG. 7C, for example). Fitting these measurements to the sinusoidalcurve gives points 704 and 706 as shown. Knowing the biases, the trueoverlay error OV can be calculated. The pitch P of the sinusoidal curveis known from the design of the target. The vertical scale or amplitudeof the curve 702 is not known to start with, but is an unknown factorwhich can be referred to as a K value. This K value is a measure of thestack sensitivity of the intensity asymmetry measurements to the target.If the determined K value is not accurate then the overlay determinedwill also be inaccurate. Furthermore, the K value may be target specificand vary across the substrate due to process variation across thesubstrate. For example K values between each target may vary due tochemical mechanical polishing or stack thickness.

In equation terms, the relationship between overlay error OV, K value,and intensity asymmetry A is assumed to be:A _(±d) =K sin(OV±d)  (1)where overlay error OV is expressed on a scale such that the targetpitch P corresponds to an angle 2π radians. Using two measurements ofgratings with different, known biases (e.g. +d and −d), the overlayerror OV can be calculated using:

$\begin{matrix}{{OV} = {{{atan}\left( {\frac{A_{+ d} + A_{- d}}{A_{+ d} - A_{- d}} \cdot {\tan(d)}} \right)}.}} & (2)\end{matrix}$

FIG. 10 depicts an example graph of stack sensitivity as a function ofincident radiation wavelength. Stack sensitivity can be understood as ameasurement of how the sensitivity of intensity asymmetry measurementchanges as the incident radiation wavelength is varied. The stacksensitivity or K value varies between different target stacks and isalso highly dependent on the wavelength of the incident radiation.Measurements taken at higher K value are more reliable, therefore thestack sensitivity or K value is indicative of target measurability. Inthe example showed in FIG. 10, a metrology target in the form of acomposite grating with, e.g., a 625 nm pitch, is illuminated withincident radiation comprising a spectrum of wavelengths and orthogonalpolarizations, and the values of stack sensitivity form swing curvesthat oscillates between, e.g., 0 and ±0.3 (arbitrary units) as thewavelength changes. Curves 1010 and 1012 are plots of average stacksensitivity with respect to incident radiation wavelength for, e.g., 0and 90 degrees of orthogonal polarizations, respectively. It should benoted that the stack sensitivity or K values presented here are forexemplary purposes only, and may vary under different radiationconditions or for different targets.

As shown in FIG. 10, it is desirable to select a specific wavelength atwhich the stack sensitivity reaches a maximum value in order to achievea more robust and reliable measurement. However, the wavelengthselection may have to be accurate in order to meet this condition, andany process variation or change in incident radiation may cause a shiftin the swing curves and stack sensitivity may no longer be at itsmaximum at the previously selected radiation conditions. For example, avariation in stack properties for thick stack devices may lead to shiftsin swing curves. The proportionality of stack sensitivity withwavelength shows decreased periodicity as a function of wavelength whenthe vertical distance between gratings used to form the target becomeslarger. This is evident with modern high-density circuitry such as 3DNAND devices, since quite substantial height steps may in fact bepresent. Stack differences between adjacent periodic structures of atarget or between adjacent targets may be a factor that adverselyaffects the accuracy of measurement, especially overlay measurement.Further details on stack differences and measurement accuracy can befound in European Patent Application EP16166614.4, which is herebyincorporated by reference herein in its entirety.

Stack difference may be understood as an un-designed difference inphysical configurations between adjacent periodic structures or targets.Stack difference causes a difference in an optical property (e.g.,intensity, polarization, etc.) of measurement radiation between theadjacent periodic structures or targets that is due to other thanoverlay error, other than intentional bias and other than structuralasymmetry common to the adjacent periodic structures or targets. Stackdifference includes, but is not limited to, a thickness differencebetween the adjacent periodic structures or targets (e.g., a differencein thickness of one or more layers such that one periodic structure ortarget is higher or lower than another periodic structure or targetdesigned to be at a substantially equal level), a refractive indexdifference between the adjacent periodic structures or targets (e.g., adifference in refractive index of one or more layers such that thecombined refractive index for the one or more layers for one periodicstructure or target is different than the combined refractive index forthe one or more layers for of another periodic structure or target eventhough designed to have a substantially equal combined refractiveindex), a difference in material between the adjacent periodicstructures or targets (e.g., a difference in the material type, materialuniformity, etc. of one or more layers such that there is a differencein material for one periodic structure or target from another periodicstructure or target designed to have a substantially same material), adifference in the grating period of the structures of adjacent periodicstructures or targets (e.g., a difference in the grating period for oneperiodic structure or target from another periodic structure or targetdesigned to have a substantially same grating period), a difference indepth of the structures of adjacent periodic structures or targets(e.g., a difference due to etching in the depth of structures of oneperiodic structure or target from another periodic structure or targetdesigned to have a substantially same depth), a difference in width (CD)of the features of adjacent periodic structures or targets (e.g., adifference in the width of features of one periodic structure or targetfrom another periodic structure or target designed to have asubstantially same width of features), etc. In some examples, the stackdifference is introduced by processing steps, such as CMP, layerdeposition, etching, etc. in the patterning process.

As mentioned above, stack difference causes a change in opticalproperties of measurement radiation between the adjacent periodicstructures or targets, therefore stack sensitivity measurements can betuned by varying target design parameters, such as grating pitch, CD, ortarget profiles, as further explained below with reference to FIG. 11.

FIG. 11 shows a first periodic structure 1101 of a target in the form ofa composite grating with a bias +d and an adjacent second periodicstructure 1106 of the target in the form of a composite grating with abias −d. A first incident measurement radiation beam 1110 is illuminatedon the first structure 1105 and the second structure 1103 of the firstperiodic structure 1101, where there is a bias +d between the firststructure 1105 and the second structure 1103. As a result, −1^(st)diffraction order signals 1130 and 1120 are diffracted by the firststructure 1105 and the second structure 1103, respectively. The −1^(st)diffraction order signal diffracted by the first periodic structure1101, I′⁻¹ ^(+d), may be understood as the combination of the −1^(st)diffraction order signals 1130 and 1120. Additionally, +1^(st)diffraction order signals 1150 and 1140 are diffracted by the firststructure 1105 and the second structure 1103, respectively. The +1^(st)diffraction order signal diffracted by the first periodic structure1101, I′₊₁ ^(+d), may be understood as the combination of the +1^(st)diffraction order signals 1150 and 1140. Accordingly, the −1 stdiffraction order signal diffracted by the first periodic structure1101, I′⁻¹ ^(+d), and the 1^(st) diffraction order signal diffracted bythe first periodic structure 1101, I′₊₁ ^(+d), may be collectivelyexpressed by:I′ _(±1) ^(+d) =C*cos(β±φ₊)  (3)where C indicates the contrast of the signal (which is a function of theperiodic structure design, measurement wavelength, etc.),

${\beta = {4\pi\frac{T}{\lambda}}},$T is the thickness of the first periodic structure, λ is the measurementradiation wavelength, phase term

${\varphi_{+} = {2\pi\frac{{OV} + d}{P}}},$OV is the actual overlay (due to any unintentional misalignment of thelayers), and P is the pitch of the first structure 1105 and the secondstructure 1103 of the first periodic structure 1101. In FIG. 12, theintensity profiles of the −1^(st) diffraction order signal diffracted bythe first periodic structure 1101, I′⁻¹ ^(+d), and the +1^(st)diffraction order signal diffracted by the first periodic structure1101, I′₊₁ ^(+d) are depicted in traces 1160 and 1170, respectivelyaccording to equation (3).

Similarly, a second incident measurement radiation beam 1115 isilluminated on the first structure 1109 and the second structure 1107 ofthe second periodic structure 1106, where there is a bias −d between thefirst structure 1109 and the second structure 1106. As a result, −1^(st)diffraction order signals 1135 and 1125 are diffracted by the firststructure 1109 and the second structure 1107 of the second periodicstructure 1106, respectively. The −1^(st) diffraction order signaldiffracted by the second periodic structure 1106, I′⁻¹ ^(−d), may beunderstood as the combination of the −1^(st) diffraction order signals1135 and 1125. Additionally, +1^(st) diffraction order signals 1155 and1145 are diffracted by the first structure 1109 and the second structure1107, respectively. The +1^(st) diffraction order signal diffracted bythe second periodic structure 1106, I′₊₁ ^(−d), may be understood as thecombination of the +1^(st) diffraction order signals 1155 and 1145.Accordingly, the −1^(st) diffraction order signal diffracted by thesecond periodic structure 1106, I′⁻¹ ^(−d), and the +1^(st) diffractionorder signal diffracted by the second periodic structure 1106, I′₊₁^(−d), may be collectively expressed by:I′ _(±1) ^(−d)=1+C*cos(β±φ⁻)  (4)where C indicates the contrast of the respective signal,

${\beta = {4\pi\frac{T}{\lambda}}},$T is the thickness of the second periodic structure, λ is themeasurement radiation wavelength, phase term

${\varphi_{-} = {2\pi\frac{{OV} - d}{P}}},$OV is the actual overlay (due to any unintentional misalignment of thelayers), and P is the pitch of the first structure 1109 and the secondstructure 1107 of the second periodic structure 1106. In FIG. 13, theintensity profiles of the −1^(st) diffraction order signal diffracted bythe second periodic structure 1106, I′⁻¹ ^(−d), and the +1^(st)diffraction order signal diffracted by the second periodic structure1106, I′₊₁ ^(−d), are depicted in traces 1180 and 1190, respectivelyaccording to equation (4).

Now, FIG. 14 illustrates a situation where a stack difference existsbetween a first periodic structure 1201 with a bias +d and an adjacentsecond periodic structure 1206 with a bias −d. In this case, the stackdifference is a different in thickness as shown in FIG. 14 and describedhereafter. Similar to FIG. 13, a first incident measurement radiationbeam 1210 is illuminated on the first structure 1205 of the firstperiodic structure 1201 and the second structure 1203 of the firstperiodic structure 1201, respectively. As a result, −1^(st) diffractionorder signals 1230 and 1220 are diffracted by the first structure 1205and the second structure 1203, respectively. Accordingly, the −1^(st)diffraction order signal diffracted by the first periodic structure1201, I′⁻¹ ^(−d), may be understood as the combination of the −1^(st)diffraction order signals 1230 and 1220. Additionally, +1^(st)diffraction order signals 1250 and 1240 are diffracted by the firststructure 1205 and the second structure 1203, respectively. Accordingly,the +1^(st) diffraction order signal diffracted by the first periodicstructure 1201, I′₊₁ ^(−d), may be understood as the combination of the+1^(st) diffraction order signals 1250 and 1240.

Similarly, a second incident measurement radiation beam 1215 isilluminated on the first structure 1209 and the second structure 1207 ofthe second periodic structure 1206, respectively. As a result, −1^(st)diffraction order signals 1235 and 1225 are diffracted by the firststructure 1209 and the second structure 1207, respectively. Accordingly,the −1^(st) diffraction order signal diffracted by the second periodicstructure 1206, I′⁻¹ ^(+d), may be understood as the combination of the−1^(st) diffraction order signals 1225 and 1235. Additionally, +1^(st)diffraction order signals 1255 and 1245 are diffracted by the firststructure 1209 and the second structure 1207, respectively. Accordingly,the +1^(st) diffraction order signal diffracted by the second periodicstructure 1206, I′₊₁ ^(+d), may be understood as the combination of the+1^(st) diffraction order signals 1255 and 1245.

As an example of stack difference, the first periodic structure 1201 andthe second periodic structure 1206 may have a difference in thickness asshown in FIG. 14. However, in another example, the stack difference maybe created by one or more other factors that allow for an additional oralternative difference in un-designed physical configuration between thefirst periodic structure 1201 and the second periodic structure 1206.For example, a stack difference may be created when the first periodicstructure 1201 is more opaque to the first measurement radiation beam1210 than the second periodic structure 1206. For example, there may bea difference in material (e.g., a same type of material having adifferent refractive index, a different type of material, etc.) betweenthe first periodic structure 1201 and the second periodic structure1206. As another example, there may be a difference in pitch of thefirst periodic structure 1201 relative to the second periodic structure1206 even though they are designed to have substantially the same pitch.These examples of stack difference are not the only ways there can be astack difference and so should not be considered as limiting.

Referring back to equations (3) and (4), the stack difference mayintroduce three additional terms in each of equations (3) and (4). Thefirst term, ΔI_(N), indicates an actual change to the intensity of therespective signal. The second term, ΔC_(N), indicates an actual changeto the contrast of the respective signal. The third term, Aβ, indicatesan actual change to the phase of the respective signal. The three termsare dependent on the wavelength and/or the polarization of themeasurement radiation beams 1210 and 1215. So, in the presence of astack difference, the −1^(st) diffraction order signal diffracted by thefirst periodic structure 1201, I′⁻¹ ^(+d), and the +1^(st) diffractionorder signal diffracted by the first periodic structure 1201, I′₊₁^(+d), may be collectively expressed by:I′ _(±1) ^(+d)=(1+ΔI _(N))*{1+C*(1+ΔC _(N))*cos[(β+Δβ)±φ₊]}.  (5)

As mentioned above, an example for stack difference or target design isa difference in pitch, i.e., pitch difference between the first periodicstructure 1201 relative to the second periodic structure 1206. Accordingto equations (1) through (5), swing curves of stack sensitivity is afunction of target design. Appropriate adjustments could be made fordifferent target design parameters, such as pitch, CD, side wall angles,target profiles etc., and multiple design parameters may be adjustedconcurrently.

FIG. 15 shows various swing curves with respect to modified targetpitches, while other target design parameters are kept constant in thisexample solely for the sake of simplicity, according to an embodiment.Using targets similar to those described in FIGS. 4, 13, and 14, acluster of targets have been fabricated with target pitches varying overa range of 600 nm and 740 nm. Curves 1501, 1503, and 1505 are chosen tobe shown here among a plurality of plots, with each curve plotted forstack sensitivity with respect to incident radiation wavelength fortarget pitches of 600 nm, 620 nm, and 640 nm, respectively. It should benoted that these pitches, wavelengths and resulting swing curves areselected only as examples and should not be considered as limiting. Astarget pitch varies, amplitude of the swing curves changes and the peaks(maximum stack sensitivity) also shifts horizontally along the x axis(incident radiation wavelength). As a result, for a specific incidentradiation wavelength, there could be a desired target design that has amaximum stack sensitivity. Alternatively, a desired incident radiationwavelength can be determined for each target design at which stacksensitivity reaches maximum. Similarly, for a specific incidentradiation polarization, there could be a desired target design that hasa maximum stack sensitivity. Alternatively, a desired incident radiationpolarization can be determined for each target design at which stacksensitivity reaches maximum.

The metrology target described above is also designed for one or moreparticular layers associated with a particular process stack (i.e., theprocess stack being the processes and material used to construct aparticular device or part thereof for the layer, e.g., the one ormaterial layers involved (e.g., the thickness and/or material typethereof), the lithographic exposure process, the resist developmentprocess, the bake process, the etch process, etc.) with the flexibilitythat the metrology target will provide measurement robustness fornominal changes in the process stack. That is, the metrology target isdesigned using knowledge of the process layers (e.g., their material,thickness, etc.), process variations, or the processing steps applied tothe layers, etc. to arrive at a metrology target that will give optimalmeasurement results for the lithographic process parameter beingmeasured.

As mentioned above, a metrology target measurement is most robust andreliable when the absolute value of stack sensitivity or K value is atmaximum for a specific incident radiation wavelength, polarization, orprocess stack.

FIG. 16 is a flow diagram of an illustrative method 1600 of improvingrobustness and measurability of metrology target stacks, in accordancewith an embodiment of this present disclosure. Other method steps may beperformed between the various steps of method 1600, and are omittedmerely for clarity. Not all steps of method 1600 described below may berequired, and in certain circumstances the steps may not be performed inthe order shown.

Method 1600 begins with step 1602 where a number of N multi-layertargets are fabricated on the wafer for use in any appropriatelithography/metrology equipment. Examples of target design andconstruction are described above with reference to FIG. 3 or 7A-7B. Thenumber of multi-layer targets N is not limited to four as shown in FIG.3 and may be selected based on measurement needs. The multi-layertargets may be clustered at one location on the wafer, or may be placedat different locations to investigate target properties across a largerarea on the wafer. The multi-layer targets' design may vary between eachtarget by modifying one or more geometrical or fabrication parameters,including, but not limited to, pitch, CD, sub-segmentation, sidewallangle, duty cycle of the line and spaces, height, width, material, etc.

Method 1600 continues with step 1604, where the multi-layer targets areilluminated with an incident illumination radiation. The incidentillumination radiation may comprise a variation of wavelengths,polarizations, or beam profiles etc. The illumination profile may bedetermined based on the metrology target design. Overlay measurementsfor each of the metrology targets are extracted from a difference in thelight intensity for the first positive and negative first diffractedorder reflected from the metrology targets.

Method 1600 continues with step 1606, at which at least a number of Nstack sensitivity values K_(i), where i∈[1, N], from the cluster ofmulti-layer targets are determined based on the overlay measurements.The determination of stack sensitivity values K_(i) may be performed bya computer processor using a computer-implemented method. As describedabove, stack sensitivity or K value may vary across the wafer due toprocess perturbations, and may be different between each of themulti-layer targets. Therefore each multi-layer target T_(i) has a Kvalue of K_(i), where i∈[1, N].

Method 1600 continues with step 1608, where the metrology parameters ofthe multi-layer target are selected or adjusted to achieve a large Kvalue. The determination of the metrology parameters may be performed bya computer processor using a computer-implemented method. The metrologyparameters may include and are not limited to geometrical or fabricationparameters, for example, pitch, CD, sub-segmentation, sidewall angle,duty cycle of the line and spaces, height, width, refraction index, etc.A simulation package may be used to select or adjust the metrologyparameters of the multi-layer target design such that a maximum K valueis achieved to provide the most robust and reliable measurement. Anexample for the simulation package may include an overarchingmethodology that is referred to as “Design for Control”, abbreviated asD4C. Further details of D4C can be found in United State PatentPublication U.S. 20160140267, which is hereby incorporated by referencein its entirety.

Based on the determination process described above with reference tomethod 1600, processing parameters of the metrology system may also becalibrated to achieve the most robust and reliable measurement. Forexample, processing parameters such as a wavelength of radiation used inthe metrology system for the target, polarization of radiation used inthe metrology system, numerical aperture of the metrology system, may beadjusted.

FIG. 17A is a flow diagram of an illustrative method 1700 of improvingrobustness and measurability of metrology target stacks, in accordancewith another embodiment of the present disclosure. Other method stepsmay be performed between the various steps of method 1700, and areomitted merely for clarity. Not all steps of method 1700 described belowmay be required, and in some circumstances the steps can be performed ina different order.

FIG. 17B is an exemplary graph of K value as a function of metrologytarget locations for different metrology target designs, according to anembodiment.

Method 1700 begins with step 1702 where a number of N multi-layertargets are fabricated on the wafer by any appropriatelithography/metrology equipment. Similar to the metrology target designdescribed in step 1602 above, the metrology parameters of themulti-layer targets may include, and are not limited to, geometrical orfabrication parameters, for example, pitch, CD, sub-segmentation,sidewall angle, duty cycle of the line and spaces, height, width,refraction index, etc. Clusters of multi-layer targets may be formed ondifferent areas across the wafer, while each area may comprise multipletargets with different designs. Therefore it is possible tosimultaneously have similarly designed targets placed across the wafersurface while also have targets with different designs placed in closeproximity at a specific region on the wafer.

Method 1700 continues with step 1704, where the multi-layer targets areilluminated with an incident illumination radiation. The incidentillumination radiation may comprise a variation of wavelengths,polarizations, or beam profiles etc., and the illumination profile maybe determined based on the metrology target design. Overlay measurementsfor each of the metrology targets are extracted from a difference in thelight intensity for the first positive and negative first diffractedorder reflected from the metrology targets.

Method 1700 continues with step 1706, at which at least a number of Nstack sensitivity values K_(i), where i∈[1, N], from the cluster ofmulti-layer targets are determined based on the overlay measurements.The determination of stack sensitivity values K_(i) may be performed bya computer processor using a computer-implemented method. As describedabove, stack sensitivity or K value may vary across the wafer due toprocess perturbations, and may be different between each of themulti-layer targets. Therefore each multi-layer target T_(i) has a Kvalue of K_(i) and assigned a multiplication factor α_(i), where i∈[1,N]. It should be noted that sensitivity values are hereby presented asexemplary modification (or optimization) parameters, and any appropriatemodification parameter with any reference value may be used, for examplebut not limited to, target coefficient or overlay error. Multiplicationfactor α₁ is a coefficient that can be modified based on processingcondition, and can be the result of any correlation analysis, such asPrincipal Component Analysis (PCA). Different methods of correlationanalysis may be used, and PCA analysis is referred to herein purely asone example. PCA is a mathematical procedure well-known in the art andneed not be discussed in detail here.

Method 1700 continues with step 1708, where the weighted sum of K_(i) isadjusted to reach a maximum value. The determination and optimization ofstack sensitivity values K_(i) may be performed by a computer processorusing a computer-implemented method. In accordance with an embodiment ofthe present disclosure, the metrology target measurement is most robustand reliable when Σ_(i=1) ^(N)α_(i)*K_(i) reaches maximum value, whileΣ_(i=1) ^(N)α_(i)=1 and α_(i)∈[−1,1]. For example, larger multiplicationfactors such as α_(i)=1 may be assigned to target measurements withhigher sensitivity values such that they are given more weight in thecalculated sum, while lower multiplication factors such as α_(i)=−1 maybe assigned to target measurements with lower sensitivity values suchthat they are given less weight in the calculated sum. Referring to FIG.17B as an example, K values are measured for various target designs atvarious locations across the wafer. At a specific location, K values1711, 1713, 1715, and 1717 are determined for metrology targets 1711′,1713′, 1715′, and 1717′ (not shown), respectively. It is to beunderstood that the K values or metrology targets herein are for thepurpose of description by example and not of limitation, and there maybe a plurality of metrology targets with different designs formed on thewafer. A large multiplication factor α_(i)=1 is assigned to K value 1711since it has a high sensitivity value. Similarly, a low multiplicationfactor α_(i)=−1 is assigned to K value 1717 since it has a lowsensitivity value. The determination of multiplication factors may alsodepend on the correlation between the optimization factor and any targetproperties, such as location, stack indicators, pitch, CD,sub-segmentation, sidewall angle, duty cycle of the line and spaces,height, width, material, etc. As mentioned above, different methods ofcorrelation analysis may be used.

Method 1700 continues with step 1710, where the metrology parameters ofthe multi-layer targets can be calculated. The final metrology parametervalue for the cluster of N multi-layer targets is a linear combinationof metrology parameter P_(i) measured for each multi-layer target, asshown in the equation below:P=Σ _(i=1) ^(N)α_(i) *P _(i)  (6)where i∈[1, N]. Therefore the final metrology parameter P can bemodified (or optimized) based on the linear combination of individualmetrology parameter P_(i) calculated from each multi-layer target andmultiplication factor α_(i).

Based on the determination process described above with reference tomethod 1700, metrology target designs, such as gratings design, can befurther modified to accommodate a variety of lithography processes andprocess perturbations, and achieve maximized robustness andmeasurability. For example, methods and systems for automaticallygenerating robust metrology targets include D4C.

Based on the determination process described above with reference tomethod 1700, processing parameters of the lithography system may becalibrated to achieve the most robust and reliable measurement. Forexample, processing parameters such as a wavelength of radiation used inthe metrology system for the target, polarization of radiation used inthe metrology system, numerical aperture of the metrology system, may beadjusted based on the stack sensitivity measurement from the metrologytargets.

FIG. 18A is a flow diagram of an illustrative method 1800 of improvingrobustness and measurability of overlay in metrology target stacks, inaccordance with a further embodiment of the present disclosure. Othermethod steps may be performed between the various steps of method 1800,and are omitted merely for clarity. Not all steps of method 1800described below may be required, and in some circumstances the steps canbe performed in a different order.

FIG. 18B is an example graph of overlay value as a function of metrologytarget locations for different metrology target designs, according to anembodiment.

Method 1800 begins with step 1802 where a number of N multi-layertargets are fabricated on the wafer for use in an overlay measurementmethod by the inspection apparatus of FIG. 3, or by any appropriatelithography/metrology equipment. Similar to the metrology target designdescribed in step 1602 above, the design may vary between eachmulti-layer target by modifying one or more geometrical or fabricationparameters, including, but not limited to, pitch, CD, sub-segmentation,sidewall angle, duty cycle of the line and spaces, height, width,refraction index, etc. As noted above, clusters of multi-layer targetsmay be formed on different areas across the wafer, while each area maycomprise multiple targets with different designs.

Method 1800 continues with step 1804, where the multi-layer targets areilluminated with an incident illumination radiation. The incidentillumination radiation may comprise a variation of wavelengths,polarizations, or beam profiles etc. The illumination profile may bedetermined based on the metrology target design. Overlay measurementsfor each of the metrology targets are extracted from a difference in thelight intensities for the positive and negative first diffraction ordersof scattered light from the metrology targets.

Method 1800 continues with step 1806, at which at least a number of Nstack sensitivity values K_(i), where i∈[1, N], from the cluster ofmulti-layer targets are determined based on the overlay measurements.The determination of stack sensitivity values K_(i) may be performed bya computer processor using a computer-implemented method. As describedabove, stack sensitivity or K value may vary across the wafer due toprocess perturbations, and may be different between each of themulti-layer targets. Therefore each multi-layer target T_(i) has a Kvalue of K_(i). As noted above, stack sensitivity values are herebypresented as exemplary modification values, and any appropriatemodification values with any reference value may be used.

Method 1800 continues with step 1808, where the overlay value OV_(i) foreach multi-layer target T_(i) is calculated by using equation (2) fromabove and

$K_{i} = {\frac{A_{+ d} - A_{- d}}{2d}.}$Each sensitivity value K_(i) is assigned a multiplication factor α_(i)^(OV), where i∈[1, N]. Multiplication factor α_(i) ^(OV) is acoefficient that can be modified based on processing condition andexternal reference overlay value, and can also be the result of anycorrelation analysis, such as Principal Component Analysis (PCA). Asmentioned above, different methods of correlation analysis may be used,and PCA analysis is referred to herein merely as one example.

Method 1800 continues with step 1810, where the weighted sum of stacksensitivity values is adjusted. The determination and optimization ofthe stack sensitivity values may be performed by a computer processorusing a computer-implemented method. In accordance with an embodiment ofthe present disclosure, the metrology target measurement is most robustand reliable when Σ_(i=1) ^(N)=α_(i) ^(OV)*K_(i) reaches maximum value.

Multiplication factors α_(i) ^(OV) can be further adjusted based onexternal reference overlay values to provide a more accurate measurementof the final overlay value. An exemplary method of optimizing (ormodifying) final overlay value OV using an external reference overlayvalue is hereby described with reference to FIG. 18B. It is to beunderstood that the overlay values or metrology targets herein are forthe purpose of description by example and not of limitation, and theremay be a plurality of metrology targets with different designs formed onthe wafer. Overlay values are measured for various target designs atvarious locations across the wafer. At a specific location, overlayvalues 1811, 1813, 1815, and 1817 are determined for targets 1811′,1813′, 1815′, and 1817′ (not shown), respectively. For example, if theoverlay values of a number of multi-layer targets T_(i) aresystematically off from an external reference overlay value 1819, eachindividual multiplication factor α_(i) ^(OV) can also be adjusted basedon the difference between the corresponding overlay value and theexternal reference overlay value. As a result, multi-layer targets thathave overlay values closer to the external reference overlay value 1819may have a relatively higher multiplication factor α_(i) ^(OV) such thatthey are given more weight in the calculated sum, while multi-layertargets that have overlay values deviate away from the externalreference overlay value may have a relatively lower multiplicationfactor α_(i) ^(OV), such that they are given less weight in thecalculated sum. For example, a low multiplication factor α_(i) ^(OV)=0.6will be assigned to overlay value 1811 since it has a relatively largedifference between external reference sensitivity value 1819. Similarly,a high multiplication factor α_(i) ^(OV)=1.2 will be assigned to overlayvalue 1815 since it has a relatively small difference between externalreference sensitivity value 1819. Therefore the final overlay value OVcan be further optimized (or modified) based on the linear combinationof individual overlay value calculations from each multi-layer targets,providing a more robust measuring process than using a single target.

Method 1800 continues with step 1812, where the final overlay value OVfor the cluster of N multi-layer targets is calculated through a linearcombination of each overlay value OV_(i) via the equation below:OV=Σ_(i=1) ^(N)α_(i) ^(OV)*OV_(i)  (6)where i∈[1, N]. Therefore the final overlay value OV can be optimized(or modified) based on the linear combination of individual overlayvalue calculations from each multi-layer targets.

Similarly, final overlay value OV can also be optimized (or modified)based on multiplication factor α_(i) and an external reference overlayvalue obtained from an external source, such as CD-SEM measurement orHolistic Metrology Qualification (HMQ) estimate. Further details of HMQcan be found in PCT Application WO 2015/018625 A1, which is herebyincorporated by reference herein in its entirety.

Based on the determination process of final overlay value OV describedabove, appropriate metrology target designs can be selected or furthermodified to accommodate a variety of lithography processes and processperturbations, and achieve maximized robustness and measurability. Forexample, methods and systems for automatically generating robustmetrology targets include D4C.

Based on the determination process described above with reference tomethod 1800, processing parameters of the lithography system may becalibrated to achieve the most robust and reliable measurement. Forexample, processing parameters such as a wavelength of radiation used inthe metrology system for the target, polarization of radiation used inthe metrology system, numerical aperture of the metrology system, may beadjusted.

FIG. 19 is a flow diagram of an illustrative method 1900 of measuring alithography process parameter using metrology targets, in accordancewith an embodiment of this present disclosure. Other method steps may beperformed between the various steps of method 1900, and are omittedmerely for clarity. Not all steps of method 1900 described below may berequired, and in certain circumstances the steps may not be performed inthe order shown.

Method 1900 begins with step 1902, where light scattered by a pluralityof metrology targets is measured. The plurality of metrology target maybe illuminated with an incident radiation, the incident radiation havingan illumination profile such as wavelength or polarization. Themeasurement of scattered light is performed in optical instruments suchas scatterometers or other metrology tools. The plurality of metrologytargets are designed using metrology parameters and produced by amanufacturing process. Examples of metrology parameters are, but notlimited to, the pitch of the gratings used to form the metrology target,CD, angle of the lines forming the gratings, duty cycle of lines andspaces forming the grating. An example of the manufacturing process is,but not limited to, a lithographic manufacturing process using alithographic projection apparatus, a pattern (e.g. in a mask) is imagedonto a substrate that is at least partially covered by a layer ofradiation-sensitive material (resist). Prior to this imaging step, thesubstrate may undergo various procedures, such as priming, resistcoating and a soft bake. After exposure, the substrate may be subjectedto other procedures, such as a post-exposure bake (PEB), development, ahard bake and measurement/inspection of the imaged features. This arrayof procedures is used as a basis to pattern an individual layer of adevice, e.g. a metrology target or an IC. Such a patterned layer maythen undergo various processes such as etching, ion-implantation(doping), metallization, oxidation, chemo-mechanical polishing, etc.,all intended to finish off an individual layer.

Method 1900 continues with step 1904, where a lithographic processparameter for the plurality of metrology targets is determined using aweighted contribution from each metrology target.

The weighted contribution from each metrology target may be determinedusing a method similar to method 1700, where modification values aredetermined for each metrology target based on their respective scatteredlight measurements, and a multiplication value is determined for eachmodification value. The multiplication factors are determined bycalculating and maximizing a sum of the multiplication factorsmultiplied by their corresponding modification values. For example,larger multiplication factors may be assigned to target measurementswith higher modification values such that they are given more weight inthe calculated sum, while lower multiplication factors may be assignedto target measurements with lower modification values such that they aregiven less weight in the calculated sum. An individual lithographicprocess parameter for each metrology target is also determined, andthese individual lithographic process parameters are used to determinethe lithographic process parameter for the plurality of metrologytargets by calculating a sum of the determined multiplication factorsmultiplied by their corresponding individual lithographic processparameters.

Alternatively, the weighted contribution from each metrology target maybe determined using a method similar to method 1800, wheremultiplication values are further determined using a referencelithographic process parameter. First, modification values aredetermined for each metrology target of the plurality of metrologytargets based on their scattered light measurements, and amultiplication value is determined for each modification value bymaximizing a sum of the multiplication factors multiplied by theircorresponding modification values. Then the multiplication factors arefurther adjusted by determining an individual lithographic processparameter for each metrology target, and adjusting the multiplicationfactors based on differences between the reference lithographic processparameter and their corresponding individual lithographic processparameters. The lithographic process parameter is then determined bydetermining a sum of the multiplication factors multiplied by theircorresponding individual lithographic process parameters.

FIG. 20 is a flow diagram of an illustrative method 2000 for metrologysystem calibration using metrology targets, in accordance with anembodiment of this present disclosure. Other method steps may beperformed between the various steps of method 2000, and are omittedmerely for clarity. Not all steps of method 2000 described below may berequired, and in certain circumstances the steps may not be performed inthe order shown.

Method 2000 begins with step 2002, where light scattered by a pluralityof metrology targets is measured. Similar to method 1900, the pluralityof metrology target may be illuminated with an incident radiation havingan illumination profile such as wavelength or polarization. Themeasurement of scattered light is performed in optical instruments suchas scatterometers or other metrology tools. The plurality of metrologytargets are designed using metrology parameters and produced by amanufacturing process.

Method 2000 continues with step 2004, where a modification value isdetermined for each metrology target using their scattered lightmeasurements. Therefore each multi-layer target has a determinedmodification value and a multiplication factor. Examples of modificationvalues include, but not limited to, stack sensitivity, targetcoefficient, or overlay error.

Method 2000 continues with step 2006, where multiplication factors aredetermined by calculating and maximizing a sum of the multiplicationfactors multiplied by their corresponding modification values. Similarto method 1700, the multiplication factors are determined by calculatingand maximizing a sum of the multiplication factors multiplied by theircorresponding modification values. For example, larger multiplicationfactors may be assigned to target measurements with higher modificationvalues such that they are given more weight in the calculated sum, whilelower multiplication factors may be assigned to target measurements withlower modification values such that they are given less weight in thecalculated sum. Measurement processes can be calibrated by using thedetermined multiplication factors and their corresponding metrologytargets.

Using the metrology targets that provide higher modification values,processing parameters of the metrology system may also be calibrated toachieve the most robust and reliable measurement. For example,processing parameters under which metrology targets with highmodification values are designed to provide the most robust measurementcan be selected for subsequent measurements. Processing parametersinclude, but not limited to, wavelengths or polarizations of incidentradiation used in the metrology system for metrology target measurement,process stack configuration, or numerical aperture of the metrologysystem.

FIG. 21 is a flow diagram of an illustrative method 2100 for metrologytarget design, in accordance with an embodiment of this presentdisclosure. Other method steps may be performed between the varioussteps of method 2100, and are omitted merely for clarity. Not all stepsof method 2100 described below may be required, and in certaincircumstances the steps may not be performed in the order shown.

Method 2100 begins with step 2102, where a plurality of metrologyparameters is provided to a computer apparatus for generating aplurality of metrology targets designs corresponding to a plurality ofmetrology targets. Similar to the metrology targets described in method1700, metrology parameters are geometrical or fabrication parameters forthe metrology targets, and examples are, but not limited to, pitch, CD,sub-segmentation, sidewall angle, duty cycle of the line and spaces,height, width, refraction index, etc. Each metrology target is designedto be respectively disposed at a different location on a substrate. Asdescribed above with reference to FIG. 5, there is a plurality ofcomposite targets placed at different locations on substrate W such thatmeasurements and information about desired areas on substrate W can beobtained. Therefore, clusters of multi-layer targets may be formed ondifferent areas across the wafer, while each area may comprise multiplesub-targets with different designs. It is possible to simultaneouslyhave similarly designed targets placed across the wafer surface whilealso have targets with different designs placed in close proximity at aspecific region on the wafer.

Method 2100 continues with step 2104, where illumination parameters forthe incident radiation used to measure the plurality of metrologytargets are received in the computer apparatus. The illuminationparameters may comprise a variation of wavelengths, polarizations, orbeam profiles etc.

Method 2100 continues with step 2106, where the computer apparatusdetermines the metrology parameters for each metrology target using theillumination parameters or process stack information of the substrate.The metrology parameters are determined such that a selection ofdifferent metrology targets will provide different measurement resultsunder incident radiations or process stack configurations. As describedabove with reference to FIG. 15, there could be a desired target designthat provides a maximum stack sensitivity for a specific illuminationparameter or process stack. For example, a target design may bedetermined such that stack sensitivity reaches maximum for a measurementusing a desired incident radiation wavelength or polarization.Similarly, a target design may be determined such that stack sensitivityreaches maximum for a desired incident process stack.

FIG. 22 schematically depicts a form of multi-grating metrology targetsdisposed on a substrate by a manufacturing process, in accordance withan embodiment of this present disclosure. Similar to the compositetargets described with reference to FIG. 5, multi-grating metrologytarget 2202 comprises at least two sub-targets positioned closelytogether so that they will all be within a measurement spot formed bythe illumination beam of the metrology apparatus. The two sub-targetshave different geometrical or fabrication parameters, for example,pitch, CD, sub-segmentation, sidewall angle, duty cycle of the line andspaces, height, width, refraction index, etc. This can be achieved byadjusting metrology parameters used to design the metrology targets.Designs of the sub-targets are determined such that stack sensitivity ofa sub-target reaches maximum for a desired incident radiation wavelengthor polarization. Similarly, a sub-target design may be determined suchthat stack sensitivity reaches maximum for a desired incident processstack. Therefore the metrology targets provide different measurementsensitivity under various incident radiations or process stackconfigurations.

A plurality of multi-grating metrology target 2202 are placed atdifferent locations on substrate W. Location choices are determinedbased on measurement needs, such as but not limited to, whethermeasurement information is needed for that location of substrate W or toeliminate process variation effects on metrology measurements, such asstack depth variation effects. Therefore it is possible tosimultaneously have similarly designed metrology targets each with aplurality of sub-targets placed across the wafer surface, while alsohave targets with different designs placed in close proximity at aspecific region on the wafer.

While the target structures described herein are metrology targetsspecifically designed and formed for the purposes of measurement, inother embodiments, properties may be measured on targets which arefunctional parts of devices formed on the substrate. Many devices haveregular, grating-like structures. The terms ‘target’, ‘target grating’and ‘target structure’ as used herein do not require that the structurehas been provided specifically for the measurement being performed.

While overlay targets in the form of gratings have been described, in anembodiment, other target types may be used such as box-in-box imagebased overlay targets.

While metrology targets to determine overlay have been primarilydescribed, the metrology targets may be used to determine, in thealternative or additionally, one of more other characteristics, such asfocus, dose, etc.

The metrology targets according to an embodiment may be defined using adata structure such as a pixel-based data structure or a polygon-baseddata structure. The polygon-based data structure may, for example, bedescribed using GDSII data formats, which are rather common in the chipmanufacturing industry. Still, any suitable data structure or dataformat may be used without departing from the scope of the embodiments.The metrology targets may be stored in a database from which a user mayselect the required metrology target for use in a particularsemiconductor processing step. Such a database may comprise a singlemetrology target or a plurality of metrology targets selected oridentified according to the embodiment. The database may also comprise aplurality of metrology targets in which the database comprisesadditional information for each of the plurality of metrology targets.This additional information may comprise, for example, informationrelated to a suitability and/or a quality of the metrology target for aspecific lithographic process step and may even include suitabilityand/or quality of a single metrology target to different lithographicprocess steps. The suitability and/or quality of the metrology targetmay be expressed in a suitability value and/or a quality value,respectively, or any other value which may be used during a selectionprocess of selecting one metrology target from the database which is tobe used for the specific lithographic process step.

In an embodiment, the computer readable medium may comprise instructionsfor activating at least some of the method steps using a connection tothe computer readable medium from a remote computer or from a remotesystem. Such connection may, for example, be generated over a securenetwork or via a (secure) connection over the world-wide-web (internet).In this embodiment, users may, for example, log in from a remotelocation to use the computer readable medium for determining asuitability and/or a quality of the metrology target design. Theproposed metrology target design may be provided by the remote computer(or by an operator using the remote computer to provide the metrologytarget design to the system for determining the suitability of themetrology target design). So the proposed metrology target design whichis to be simulated using models may be owned by a different entity orcompany compared to the models used during the simulation process.Subsequently, the resulting determined suitability to evaluate themetrology target quality may be provided back to the remote computer,for example, without leaving any residual details to excess the proposedmetrology target design or the simulation parameters used. In such anembodiment, a customer may acquire the option to run an assessment ofindividually proposed metrology target designs without owning thesoftware or having a copy of the software at its remote location. Suchoption may be obtained by, for example, a user agreement. A benefit ofsuch user agreement may be that the models used in the simulations mayalways be the most recent and/or the most detailed models availablewithout the need to locally update any software. Furthermore, byseparating the model simulation and the proposed metrology targetproposal, the details of the designed markers or the different layersused for the processing need not to be shared by the two companies.

In association with the physical grating structures of the targets asrealized on substrates and patterning devices, an embodiment may includea computer program containing one or more sequences of machine-readableinstructions describing a method of designing a target, producing atarget on a substrate, measuring a target on a substrate and/oranalyzing measurements to obtain information about a lithographicprocess. This computer program may be executed for example within unitPU in the apparatus of FIGS. 3 and 4 and/or the control unit LACU ofFIG. 2. There may also be provided a data storage medium (e.g.,semiconductor memory, magnetic or optical disk) having such a computerprogram stored therein. Where an existing apparatus, for example of thetype shown in FIGS. 1-4, is already in production and/or in use, anembodiment can be implemented by the provision of updated computerprogram products for causing a processor of the apparatus to perform amethod as described herein.

An embodiment of the invention may take the form of a computer programcontaining one or more sequences of machine-readable instructionsdescribing a method as disclosed herein, or a data storage medium (e.g.semiconductor memory, magnetic or optical disk) having such a computerprogram stored therein. Further, the machine readable instruction may beembodied in two or more computer programs. The two or more computerprograms may be stored on one or more different memories and/or datastorage media.

Any controllers described herein may each or in combination be operablewhen the one or more computer programs are read by one or more computerprocessors located within at least one component of the lithographicapparatus. The controllers may each or in combination have any suitableconfiguration for receiving, processing, and sending signals. One ormore processors are configured to communicate with the at least one ofthe controllers. For example, each controller may include one or moreprocessors for executing the computer programs that includemachine-readable instructions for the methods described above. Thecontrollers may include data storage medium for storing such computerprograms, and/or hardware to receive such medium. So the controller(s)may operate according the machine readable instructions of one or morecomputer programs.

The disclosure may further be described using the following clauses:

I. A method of metrology target design, the method comprising:

receiving an illumination parameter for measuring a metrology target andselecting and/or adjusting a metrology parameter associated with themetrology target design for enhancing an accuracy and/or a robustness ofthe measurement of the metrology target design using the illuminationparameter.

II. A method to determine a parameter of a lithographic processcomprising:

receiving the light scattered from a region comprising at least twometrology targets optimized to provide a robust and optimal metrologymeasurement and

determining the parameter of the lithographic process from a weightedcontribution of each individual metrology targets.

Further embodiments according to the present invention are furtherdescribed in below numbered clauses:

1. A method comprising:

measuring light scattered by a plurality of metrology targets, theplurality of metrology targets having been designed using metrologyparameters and produced by a manufacturing process; and

determining a lithographic process parameter for the plurality ofmetrology targets using a weighted contribution from each metrologytarget.

2. The method of clause 1, wherein the weighted contribution iscalculated by determining a modification value and a multiplicationfactor for each metrology target.

3. The method of clause 2, wherein determining the multiplicationfactors further comprises determining a sum of the multiplicationfactors multiplied by their corresponding modification values.

4. The method of clause 3, wherein determining the lithographic processparameter comprises adjusting the multiplication factors such that thesum is maximized.

5. The method of clause 4, wherein determining the lithographic processparameter further comprises determining an individual lithographicprocess parameters for each metrology target, and determining a sum ofthe multiplication factors multiplied by their corresponding individuallithographic process parameters.

6. The method of clause 1, wherein the modification values are targetcoefficients or overlay errors of the plurality of metrology targets.

7. The method of clause 1, wherein the modification values are stacksensitivity values of the plurality of metrology targets.

8. The method of clause 1, wherein the lithographic process parametersare overlay values.

9. The method of clause 1, wherein the metrology parameters comprisematerial choice, critical dimension, sub-segmentation, or sidewallangle.

10. The method of clause 1, wherein the plurality of metrology targetscomprise multi-layer periodic structures.

11. The method of clause 10, wherein the metrology parameters of themulti-layer periodic structures comprise pitch, duty cycle of lines andspaces, height, or width.

12. The method of clause 1, wherein the plurality of metrology targetsare designed for different wavelengths or polarizations of an incidentradiation or process stacks.

13. The method of clause 1, wherein determining the lithographic processparameter further comprises using a reference lithographic processparameter.

14. The method of clause 13, wherein the weighted contribution iscalculated by determining a modification value and a multiplicationfactor for each metrology target.

15. The method of clause 14, wherein the multiplication factors aredetermined using a sum of the multiplication factors multiplied by theircorresponding modification values.

16. The method of clause 15, wherein the multiplication factors arefurther determined by determining an individual lithographic processparameter for each metrology target, and adjusting the multiplicationfactors based on differences between the reference lithographic processparameter and their corresponding individual lithographic processparameters.

17. The method of clause 16, wherein determining the lithographicprocess parameter further comprises determining a sum of themultiplication factors multiplied by their corresponding individuallithographic process parameters.

18. A method comprising:

measuring light scattered by a plurality of metrology targets, theplurality of metrology targets having been designed using metrologyparameters and produced by a manufacturing process;

determining a modification value for each metrology target; and

determining a multiplication factor for each metrology target based onits corresponding modification value.

19. The method of clause 18, wherein determining the multiplicationfactors comprises determining a sum of the multiplication factorsmultiplied by their corresponding modification values.

20. The method of clause 19, wherein determining the multiplicationfactors further comprises adjusting the multiplication factors such thatthe sum is maximized.

21. The method of clause 18, wherein the modification values are targetcoefficients or overlay errors of the plurality of metrology targets.

22. The method of clause 18, wherein the modification values are stacksensitivity values of the plurality of metrology targets.

23. The method of clause 18, wherein the lithographic process parametersare overlay values.

24. The method of clause 18, wherein the metrology parameters comprisematerial choice, critical dimension, sub-segmentation, or sidewallangle.

25. The method of clause 18, wherein the plurality of metrology targetscomprise multi-layer periodic structures.

26. The method of clause 25, wherein the metrology parameters of themulti-layer periodic structures comprise pitch, duty cycle of lines andspaces, height, or width.

27. The method of clause 18, wherein the plurality of metrology targetsare designed for different wavelengths or polarizations of an incidentradiation or process stacks.

28. A method of metrology target design, the method comprising:

providing a plurality of metrology parameters for generating a pluralityof metrology targets designs corresponding to a plurality of metrologytargets, wherein each metrology target is designed to be respectivelydisposed at a different location on a substrate;

receiving illumination parameters for measuring the plurality ofmetrology targets; and

determining, by a computer apparatus, the plurality of metrologyparameters for each metrology target using the illumination parametersor process stack information of the substrate.

29. The method of clause 28, wherein the illumination parameterscomprise wavelength values or polarizations of an incident radiation.

30. The method of clause 28, wherein at least one metrology target isdesigned for a different illumination parameter.

31. The method of clause 28, wherein at least one metrology target isdesigned for a different process stack.

32. The method of clause 28, wherein at least one metrology target isdesigned for different wavelengths or polarizations of an incidentradiation.

33. A metrology target, comprising:

-   -   a plurality of metrology targets disposed at different locations        on a substrate by a manufacturing process, wherein each        metrology target comprises at least first and second metrology        sub-targets that are different in design.

34. The metrology target of clause 33, wherein the first and secondmetrology sub-targets are designed for different process stacks.

35. The metrology target of clause 33, wherein the first and secondmetrology sub-targets are designed for different illuminationparameters.

36. The metrology target of clause 35, wherein the illuminationparameters comprise wavelength values or polarizations of an incidentradiation.

Although specific reference may have been made above to the use ofembodiments in the context of optical lithography, it will beappreciated that an embodiment of the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography, atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

Further, although specific reference may be made in this text to the useof lithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below. For example, one or more aspects ofone or more embodiments may be combined with or substituted for one ormore aspects of one or more other embodiments as appropriate. Therefore,such adaptations and modifications are intended to be within the meaningand range of equivalents of the disclosed embodiments, based on theteaching and guidance presented herein. It is to be understood that thephraseology or terminology herein is for the purpose of description byexample, and not of limitation, such that the terminology or phraseologyof the present specification is to be interpreted by the skilled artisanin light of the teachings and guidance. The breadth and scope of theinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

What is claimed is:
 1. A metrology target, comprising: a plurality ofmetrology targets disposed at different locations on a substrate by amanufacturing process, wherein each metrology target comprises at leastfirst and second metrology sub-targets that are different in design, andwherein the first and second metrology sub-targets are designed suchthat a stack sensitivity of each metrology sub-target reaches a maximumfor a selected incident radiation or a selected process stackconfiguration.
 2. The metrology target of claim 1, wherein the first andsecond metrology sub-targets are designed for different process stacks.3. The metrology target of claim 1, wherein the first and secondmetrology sub-targets are designed for different illuminationparameters.
 4. The metrology target of claim 3, wherein the illuminationparameters comprise wavelength values or polarizations of an incidentradiation.
 5. A method of metrology target design, the methodcomprising: providing a plurality of metrology parameters for generatinga plurality of metrology target designs corresponding to a plurality ofmetrology targets, wherein each metrology target is designed to berespectively disposed at a different location on a substrate; receivingillumination parameters for measuring the plurality of metrologytargets; determining, by a computer apparatus, one or more values of theplurality of metrology parameters for each metrology target using theillumination parameters or process stack information of the substrate,wherein the one or more values of the plurality of metrology parametersare determined such that a maximum stack sensitivity is achieved formeasurement using a selected incident radiation or a selected processstack configuration; and producing the plurality of metrology targetsbased on the determining of the one or more values of the plurality ofmetrology parameters for each of the metrology target designs.
 6. Themethod of claim 5, wherein the illumination parameters comprisewavelength values or polarizations of an incident radiation.
 7. Themethod of claim 5, wherein at least one metrology target is designed fora different illumination parameter.
 8. The method of claim 5, wherein atleast one metrology target is designed for a different process stack. 9.The method of claim 5, wherein at least one metrology target is designedfor different wavelengths or polarizations of an incident radiation. 10.The method of claim 5, wherein the metrology parameters comprisematerial choice, critical dimension, sub-segmentation, or sidewallangle.
 11. The method of claim 5, wherein the plurality of metrologytargets comprise multi-layer periodic structures.
 12. The method ofclaim 11, wherein the metrology parameters of the multi-layer periodicstructures comprise pitch, duty cycle of lines and spaces, height, orwidth.
 13. A non-transitory computer program product comprisingmachine-readable instructions which, when run on a suitable processor,cause the processor to perform operations comprising: providing aplurality of metrology parameters for generating a plurality ofmetrology target designs corresponding to a plurality of metrologytargets, wherein each metrology target is designed to be respectivelydisposed at a different location on a substrate; receiving illuminationparameters for measuring the plurality of metrology targets; determiningone or more values of the plurality of metrology parameters for eachmetrology target using the illumination parameters or process stackinformation of the substrate, wherein the one or more values of theplurality of metrology parameters are determined such that a maximumstack sensitivity is achieved for measurement using a selected incidentradiation or a selected process stack configuration; and producing theplurality of metrology targets based on the determining of the one ormore values of the plurality of metrology parameters for each of themetrology target designs.
 14. The non-transitory computer programproduct of claim 13, wherein the illumination parameters comprisewavelength values or polarizations of an incident radiation.
 15. Thenon-transitory computer program product of claim 13, wherein at leastone metrology target is designed for a different illumination parameter.16. The non-transitory computer program product of claim 13, wherein atleast one metrology target is designed for a different process stack.17. The non-transitory computer program product of claim 13, wherein atleast one metrology target is designed for different wavelengths orpolarizations of an incident radiation.
 18. The non-transitory computerprogram product of claim 13, wherein the metrology parameters comprisematerial choice, critical dimension, sub-segmentation, or sidewallangle.
 19. The non-transitory computer program product of claim 13,wherein the plurality of metrology targets comprise multi-layer periodicstructures.
 20. The non-transitory computer program product of claim 19,wherein the metrology parameters of the multi-layer periodic structurescomprise pitch, duty cycle of lines and spaces, height, or width.